An antibiotic and antiprotozoal (for anaerobic and some parasitic infections)
Metronidazole
An antibiotic used for infections caused by anaerobic bacteria and certain parasites — including dental infections, some gut and pelvic infections, bacterial vaginosis and rosacea — with a strong warning to avoid alcohol.
What is Metronidazole?
Metronidazole treats infections caused by anaerobic bacteria (those that grow without oxygen) and certain parasites — for example dental abscesses, bacterial vaginosis, some pelvic and abdominal infections, and (as a cream/gel) rosacea. The most important practical point is to avoid alcohol during treatment and for a couple of days after, because the combination can cause an unpleasant reaction with flushing, nausea and vomiting.
Education and reference only. This is a plain-language guide to Metronidazole — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Metronidazole is an antibiotic with a particular strength against anaerobic bacteria and some single-celled parasites (protozoa). It is used for a range of infections — dental infections, bacterial vaginosis and some other gynaecological and abdominal infections, certain gut infections, and skin conditions such as rosacea (usually as a cream or gel). It comes as tablets, a liquid, a skin preparation, a vaginal gel and, in hospital, an injection. Its stand-out practical feature is that alcohol must be avoided during and shortly after a course.
How it works
Once inside anaerobic bacteria or protozoa, metronidazole is chemically activated and damages the organism’s DNA, killing it. Because this activation only happens efficiently in organisms that live without oxygen, metronidazole targets anaerobes and certain parasites while leaving most of the body’s normal, oxygen-using bacteria largely alone.
What it treats
Conditions Metronidazole is used for
Practical use
How to take Metronidazole
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Avoid all alcohol during the course and for at least 48 hours afterwards — including alcohol-containing medicines and some mouthwashes.
- Take tablets with or after food to reduce nausea, and swallow them whole with water.
- Complete the whole course as prescribed, even once you feel better.
- For the vaginal gel or skin preparations, follow the specific application instructions.
- Report new numbness, tingling or unsteadiness, especially on longer courses.
Weighing it up
Advantages & disadvantages of Metronidazole
Advantages
- Highly effective against anaerobic bacteria and certain parasites that many other antibiotics miss.
- Available in many forms — tablets, liquid, skin gel/cream, vaginal gel and injection.
- Well established and inexpensive.
Disadvantages
- Must not be combined with alcohol (during and shortly after treatment).
- Commonly causes a metallic taste and mild nausea.
- Rarely affects the nerves with long or repeated use.
Practical use
Good to know
Do not drink alcohol while taking metronidazole, and for at least 48 hours after finishing — the combination can cause a "disulfiram-like" reaction with flushing, throbbing headache, nausea, vomiting and a racing heartbeat. This also applies to some medicines and mouthwashes that contain alcohol. Take the full course even if you feel better. A metallic taste and mild digestive upset are common and harmless. Long or repeated courses are used cautiously because, rarely, metronidazole can affect the nerves; tell your doctor about any new numbness or tingling.
Who should not take it / use with caution
- People who have drunk alcohol, or taken alcohol-containing preparations, within the treatment window.
- Used with care in significant liver disease and in some blood or neurological conditions.
- Pregnancy and breastfeeding — used when the benefit is judged to outweigh any risk, on medical advice (high one-off doses are usually avoided).
Monitoring
- Usually none for a short course
- Blood count and liver function on prolonged courses
- Nerve symptoms with long or repeated use; INR if on warfarin
Side effects
- Common: a metallic or unpleasant taste, nausea, loss of appetite, and furring of the tongue.
- The alcohol reaction: flushing, headache, nausea, vomiting and palpitations if alcohol is taken.
- Uncommon or with long use: darkened urine (harmless), and, rarely, numbness or tingling in the hands or feet, or effects on the blood — report these.
Key interactions
- Alcohol — avoid completely during and for 48 hours after treatment.
- It can increase the effect of warfarin (raising the INR and bleeding risk) — the INR is monitored.
- It can raise the level of lithium and some other medicines; tell your prescriber what else you take.
Available as: Tablets, oral liquid, a skin cream/gel (for rosacea), a vaginal gel, and an injection used in hospital.
Answers
Metronidazole: frequently asked questions
Why can’t I drink alcohol with metronidazole?
Metronidazole and alcohol together can cause a very unpleasant reaction — flushing, a throbbing headache, nausea, vomiting and a fast heartbeat. Avoid all alcohol during the course and for at least 48 hours after finishing, and watch out for alcohol hidden in some medicines and mouthwashes.
Why does everything taste metallic?
A metallic or altered taste is a very common and harmless effect of metronidazole that goes away after the course finishes. Taking the tablets with food and staying well hydrated can help; it is not a sign of anything wrong.
Do I need to finish the whole course?
Yes. Keep taking it for the full length prescribed, even once you feel better, to make sure the infection is properly cleared and to reduce the chance of it coming back or of resistance developing.
Is the rosacea cream the same as the tablets?
They contain the same active ingredient, but the cream or gel is applied to the skin for rosacea and delivers very little into the body, so the alcohol warning is far less of an issue with the topical form than with tablets. Follow the specific instructions for whichever you are given.
Authoritative sources
- BNF: Metronidazole.
- electronic Medicines Compendium (SmPC): Metronidazole (Flagyl).
- NICE CKS: Bacterial vaginosis; Rosacea; Dental abscess.
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